What do you do when you're 15 and have a five inch penis and a girl wants to give you oral sex and doesn't know your size when erect. Should you let her do it or be embarrassed?

Size does not matter. However, your age does matter, and in most countries anyone under the age of 16 is comitting a sexual offense while some countries (and even some specific states in the United States, or territories etc.) have to be over 16 - it all depends so it's best to do research on that specific location.